To improve the outcome of trauma victims and of patients undergoing high-blood-loss surgical procedures and to avoid the many serious complications of blood transfusions, there is a critical need for an oxygen- carrying blood substitute. This study is aimed at engineering novel hemoglobin (Hb) and actin(Ac) containing liposomes (LEAcHb). These vesicles are expected to have increased half-lives in circulation via two key features of this novel system. First these liposomes are composed of lipid that is conjugated with poly(ethylene glycol) (PEG). This feature reduces recognition of these liposomes by the reticuloendothelial system (RES). Secondly, these vesicles are both mechanically stable and able to adopt an ellipsoidal shape in solution, since the cytoskeletal polymer actin acts as a scaffold and stabilizes the vesicles while in the systemic circulation. Hence, they are highly resilient to blood shear gradients. In fact these novel vesicles can dynamically self-heal themselves when exposed to high shear stresses encountered in the blood stream. The objective of this application is to understand how LEAcHb vesicle structure and mechanics can be engineered to create mechanically stable and shape changing vesicular dispersions for use as artificial blood substitutes. We postulate that control of liposome structure and mechanics can be engineered by varying liposome size and concentration of encapsulated actin. We plan to test our hypothesis and accomplish the objective of this application by pursuing three specific aims: 1) Determine the structure and mechanics of individual actin-hemoglobin containing liposomes, and relate this to the rheology and average bending elasticity of actin-hemoglobin containing liposome dispersions. 2) Determine the oxygen binding properties at equilibrium, kinetics of O2 and NO binding/release, kinetics of Hb autoxidation and hemin release, kinetics of H2O2 and O2*~ mediated oxidation of Hb, stability, complement activation, in vivo performance, circulatory half-life, and biodistribution of LEAcHb dispersions. 3) Physiological assessment of the hemodynamic effects of various blood volume replacement regimens using LEAcHb dispersions. The proposed work is both innovative and significant, because novel LEAcHb hybrid vesicles with engineered structural and mechanical characteristics will be created for use as an artificial blood substitute.